Method of protecting a metallic surface from corrosion and resultant article

ABSTRACT

A corrosion protection system for metal. The metal surface to be protected is first cleaned of dirt, grease and the like, preferably by grit blasting. The cleaned surface is then coated first with a primer to promote adhesion between the metal and the basic protection coating. A protective top coat, preferably an epoxy, is applied over the primer either by spraying or fluid bed. Finally, an impact resistant over coat is applied over the epoxy top coat preferably by spraying to prevent damage to the top coat during handling.

United States Patent 1191 1111 3,762,939

Hunter 1 1 Oct. 2, 1973 54] METHOD or PROTECTING A METALLIC 3,023,1242/1962 Cryderman et al. 117/75 x SURFACE FROM CORROSION AND 3,305,8122/1969 Macchia et al. 336/90 RESULTANT ARTICLE 3,599,134 8/1971 Galloway336/90 lnventor: Richard F. Hunter, Hickory, NC.

Assignee: General Electric Company Filed: July 2, 1971 Appl. No.:159,409

US. Cl 117/75, 117/49, 117/94, 117/132 BE, 336/90 Int. Cl. B32b 15/08,HOlf 27/02 Field of Search 117/75, 49, 94, 132 BE; 336/90 ReferencesCited UNITED STATES PATENTS 2/1958 Lowell 117/75 8/1960 Blake 117/75 XPrimary Examiner-Ralph Husack Alt0rneyFrancis X. Doyle et al.

[57] ABSTRACT A corrosion protection system for metal. The metal surfaceto be protected is first cleaned of dirt, grease and the like,preferably by grit blasting. The cleaned surface is then coated firstwith a primer to promote adhesion between the metal and the basicprotection coating. A protective top coat, preferably an epoxy, isapplied over the primer either by spraying or fluid bed. Finally, animpact resistant over coat is applied over the epoxy top coat preferablyby spraying to prevent damage to the top coat during handling.

4 Claims, 2 Drawing Figures POLYV/NYL METHOD OF PROTECTING A METALLICSURFACE FROM CORROSION AND RESULTANT ARTICLE BACKGROUND OF THE INVENTIONThis invention relates to metal coating and more par ticularly to ametal corrosion protection system.

As is well known, many types of metal structures are exposed to theenvironment and are subject to corrosion from such environment. Somestructures are often exposed to a particularly hostile environment, suchas sea water, sea air and the like. Examples of metal structures whichmay be exposed to the environment are bridges, ships, pipe lines and, ofparticular importance to this invention, buried or subsurfacetransformers. While all metallic structures are subject to corrosion,most are provided with periodic maintenance, especially when exposed tohostile environments to prevent severe damage to the metal structurefrom corrosion. However, subsurface or buried structures in theelectrical utility field, such as subsurface transformers are usuallyexpected to operate over an extended period which may be, for example,up to 30 years, with little or no maintenance and frequently at elevatedtemperatures. This expectation places stringent requirements on anycorrosion protection system which is used to prevent the corrosion ofthe metal tanks of these subsurface transformers. A number of finish orcoating systems for protection of metal from corrosion are presentlyavailable. Many are used on subsurface transformers. However, allpresently known systems have recognized weaknesses which have causedmany manufacturers to use stainless steel tanks for subsurfacetransformers. As will be understood, stainless steel tanks addsubstantially to the cost of manufacturing of these devices. This hasbeen considered necessary by some manufacturers since most finisheseither deteriorate due to the hostile environment underground or elseare so high priced and difficult to apply in required thicknesses thatthey are considered economically unattractive. As is well known, manysynthetic coatings are hydrolytically unstable or have a highpermeability to water. Many also have low impact resistance or poorabrasion resistance leaving the metal substantially unprotected aftersustaining blows or abrasion. Thus,

: there is presently a need in the metal coating field, and

particularly with reference to subsurface transformer tanks, for acoating or finish system which will protect the metal surface fromcorrosion for a substantial period of time.

It is, therefore, one object of this invention to provide a novel metalcorrosion protection system.

A still further object of this invention is to provide a novel method ofcoating a metallic surface to prevent the surface from corroding.

A still further object of this invention is to provide a novelprotective coating system utilizing a plurality of coats.

A still further object of this invention is to provide a novelprotective coating system for subsurface transformer tanks.

SUMMARY OF THE INVENTION In carrying out this invention in a preferredform, a metal member to be protected is first cleaned of all dirt,grease and the like. A primer coating compatible both to the cleanedmetal and to a barrier top coat is then applied to the clean metalmember. A top coat of thermosetting resinuous material is coated overthe primer and cur ed to provide basic corrosion protection. An overcoat of resinous material having good impact resistance is then coatedover the thermosetting resinous material coat providing impactprotection to such coat.

The invention sought to be protected will be particularly pointed outand distinctly claimed in the claims appended hereto. However, it isbelieved that this invention and the manner in which its various objectsand advantages are obtained as well as other objects and advantagesthereof will be better understood by reference to the following detaileddescription of the present preferred embodiment, particularly when,consid ered with the accompanying drawing.

BRIEF DESCRIPTION OF DRAWING FIG. I is a plan view of a subsurfacedistribution transformer buried in the earth and utilizing the corrosionprotection system of this invention; and

FIG. 2 is a sectional view on an enlarged scale taken on the line 2-2 ofFIG. 1 showing a portion of the tank and the details of the preferredform of metallic corrosion protection system of this invention.

DESCRIPTION OF PREFERRED EMBODIMENT In the preferred form of thisinvention, a metal surface, such as the surface of a transformer tank,which is to be protected from corrosion, is first cleaned and thenprovided with a primer coat. A top barrier coat and an over coat ofvarious compatible synthetic resins are then applied to the primercoated metal, each resin performing a desired function so as to providethe best possible protection of the metal surface.

A particular type of metal surface needing protection is shown inFIG. 1. As is there shown, a transformer tank 10 is buried in earth 12providing subsurface electrical power distribution in a well knownmanner. The transformer tank 10 is usually made of a mild steel provided with a corrosion protection coating or-finish thereon. As earliernoted, buried transformers are often subjected to severe hostileenvironmental conditions, and present day protective coatings do notprovide desired maintenance free protection which is needed for suchsubsurface transformers. This invention provides a novel, long life,maintenance free corrosion protection system. The preferred form of thecorrosion protection system of this invention is best seen in FIG. 2,which is an enlarged sectional view of a portion of the transformer tankof FIG..1.

As can be seen in FIG. 2, transformer tank 10 comprises a metal wallmember 14 with its outer surface 16 being first coated with a primer l8.Primer I8 is shown as being coated with the epoxy top coat 20 whichprovides the primary corrosion protection of surface 16. A final overcoat 22 of a polyvinyl is applied over the epoxy top coat 20 to provideimpact protection to the epoxy coating 20.

In carrying out this invention, the surface to be pro tected such assurface 16 of metal 14 must be thoroughly cleaned. All oil, grease,dirt, rust and the like must be removed from the surface. In thepreferred embodiment, this is done by grit blast cleaning, althoughother types of cleaning may be used. After the surface 16 is completelycleaned, a primer 18 is coated over the entire surface 16. In thepreferred embodiment, a solvent spray is used and the sprayed primer isan epoxy resin having metal salts, such as zinc chromate or lead oxideadded thereto, and a polyamide hardener.

As will be understood, the metal salts provide additional corrosionprotection to metal surface 16. Also, fillers are provided to the primerto make its coefficient of expansion substantially the same as metal 14to prevent loss of adhesion during expansion and contraction of metal14. The preferred thickness of the primer is approximately 2 mils,although primer coatings as thin as 0.25 mils have been found adequate.As will be understood, the basic function of the primer is to provide atransition layer between the metalsurface and the top coat promotingadhesion of such top coat. Thus it will be understood that the primermust be compatible both with the metal surface and the top coat. Since,in the preferred embodiment, an epoxy top coat is used, an epoxy primeris also used.

In the preferred form of the invention, the primer is allowed to air dryfor approximately one hour to insure removal of the solvent. The primeris cured at the same time as the epoxy top coat, although it could becured before adding the top coat, if desired. The primer is acommercially available material, compatible with top coat 20.

The epoxy top coat 20 is a commercially available epoxy coating which isthinned with a solvent so as to enable the solvent spraying of the topcoat over the primer. The preferred solvent epoxy spray material is soldby the Glidden Company under the trademark NU-PON. Similar types ofsolvent spray epoxy materials are available from other commercialsources. The solvent in the epoxy spray material goes off at roomtemperature and the epoxy may then be cured for one hour at 140 C.

In the preferred embodiment, the epoxy to be coated is sprayed to athickness of between 5 and approximately mils. It has been found thatfor adequate corrosion protection at least 5 mils or epoxy top coat mustbe provided. More than 10 mils of epoxy is considered uneconomical, dueto the added cost of material and extra time and expense to obtain athicker coating.

After the epoxy top coat has been cured, an over coat ofpolyvinylchloride is then solvent sprayed over the epoxy material. Thepolyvinylchloride spray material is also available commercially, apreferred material being obtained from Sherwin-Williams Company. Thepolyvinylchloride material is sprayed to a thickness of to mils and isthen cured at a temperature of 125 C. for approximately 20 minutes. Thepolyvinylchloride material has good impact resistance prior to aging andprovides a protection to the epoxy primary coating during the storageand handling of the coated metal, such as, for example, a coatedtransformer tank.

As will be apparent from the above description, many types ofthermoplastic material could be utilized for an over coat as long asthey are sufficiently compatible with the epoxy to provide good adhesionthereto and had good impact resistance for at least the period of timerequired for handling, storing and shipping of the coated metal device.In the preferred embodiment polyvinylchloride has been selected due toits compatibility with the epoxy and its cost.

While there has been shown and described the present preferredembodiment of the corrosion protection system of this invention, it willbe understood that such system may be applied to any metallic surfacewhich may be subject to hostile corrosive environments. Further, it willbe understood that the invention herein set forth is particularlydescribed as to its limits by the attached claims.

What is claimed as new and which it is desired to secure by LettersPatent of the United States is:

l. A method of protecting a metallic surface from corrosion comprising:

a. thoroughly cleaning said metallic surface,

b. applying a thin epoxy primer coat to said surface within the range of0.25 to 2 mils,

c. applying to said primer coated surface an epoxy coating within therange of 5 to 10 mils in thickness and curing said coating and saidprimer coat, and then d. applying an over coat of polyvinyl chloridewithin the range of 15 to 20 mils in thickness and curing said polyvinylchloride over coat.

2. A method of protecting a metallic surface from corrosion as set forthin claim 1 in which said primer coat is a solvent spray epoxy materialand said primer coat is air dried for approximately one hour beforeapplying said epoxy coating.

3. A metal surface having a corrosion protection system thereoncomprising a primer coating of epoxy firmly adhering to said metalsurface, a primary corrosion coating of epoxy material in the range of 5to 10 mils in thickness adhering to said primer coating and a polyvinylchloride over coat in the range of 15 to 20 mils thick bonded to saidepoxy primary coating.

4. A distribution transformer tank suitable for subsurface distributionhaving a corrosion protection coating system comprising an epoxy primerbonded to the metal surface of the transformer tank, an epoxy primarycoat in the range of 5 to 10 mils in thickness bonded to said primer anda polyvinyl chloride over coat in the range of 15 to 20 mils inthickness bonded to said epoxy primary coat.

2. A method of protecting a metallic surface from corrosion as set forthin claim 1 in which said primer coat is a solvent spray epoxy materialand said primer coat is air dried for approximately one hour beforeapplying said epoxy coating.
 3. A metal surface having a corrosionprotection system thereon comprising a primer coating of epoxy firmlyadhering to said metal surface, a primary corrosion coating of epoxymaterial in the range of 5 to 10 mils in thickness adhering to saidprimer coating and a polyvinyl chloride over coat in the range of 15 to20 mils thick bonded to said epoxy primary coating.
 4. A distributiontransformer tank suitable for subsurface distribution having a corrosionprotection coating system comprising an epoxy primer bonded to the metalsurface of the transformer tank, an epoxy primary coat in the range of 5to 10 mils in thickness bonded to said primer and a polyvinyl chlorideover coat in the range of 15 to 20 mils in thickness bonded to saidepoxy primary coat.